Somalia's extremist group al Shabaab set off a suicide car bomb at a security checkpoint about 15 km (nine miles) outside the capital Mogadishu on Thursday, the group's spokesman and a police official told Reuters.

The blast has caused casualties, police officer Major Abdullahi Nur said without elaborating on their severity.

A witness Mohamed Farah, who lives in an area near the blast, told Reuters he heard a huge explosion, gunfire and then saw large clouds of smoke rise up from the scene of the attack.

Nur said many vehicles were parked at the Sinka Dheer checkpoint for inspection when the blast occurred.

‘There are casualties ... we shall give details after investigation,’ he said.

Al Shabaab frequently carries out bombings in Mogadishu and elsewhere across Somalia as part of their war to defeat the western-backed federal government.

They are also keen to drive out the African Union (AU)-backed AMISOM peacekeeping forces defending the central government.

Abdiasis Abu Musab, al Shabaab’s military operations spokesman, told Reuters the group was responsible for the blast, adding there were ‘many security forces’ at the checkpoint.